In the past, there has been provided an image-forming apparatus with an electrophotographic method for forming an image. In this apparatus, a surface of an electrophotographic photoreceptor is charged, then exposing the charged photoreceptor to form an electrostatic latent image on the photoreceptor. Then, the formed electrostatic latent image is developed with a developer, followed by transferring this to a paper to form an image.
Generally, an electrophotographic photoreceptor (hereafter, it may be called as “a photoreceptor”) includes a conductive support laminated thereon: an intermediate layer, a charge generating layer, a charge transport layer, and a protective layer. In recent years, the photoreceptor thus composed has been required to have longer lifetime and higher image quality. In particular, the photoreceptor has been required to be excellent in memory resistive property and abrasion resistive property for enabling to use for a long period of time. In order to achieve these properties, it was proposed a technology in which a curable binder resin, N-type metal oxide particles, and a charge transport material (CTM) were included in the protective layer, for example (refer to Patent document 1: JP-A 2013-61625).
However, according to the above-described technology, the charge transport property of the charge transport agent included in the protective layer is insufficient and a sufficient memory resistive property cannot be obtained. In order to ensure a sufficient memory resistive property, it is desirable to incorporate a charge transport agent having a high charge transport property in the protective layer.
Here, the charge transport agent usually has a photo absorption wavelength in the region of less than 400 nm. The charge transport agent having a high charge transport property will have an extensive π conjugated system. Due to extension of the π conjugated system, the absorption wavelength will be shifted to a longer side. Therefore, when a charge transport agent having a high charge transport property is incorporated in the protective layer, the UV rays to cure the curable binder resin will be absorbed by the charge transport agent. Consequently, a polymerization ratio of the resin composing the protective layer will be lowered to result in decreasing hardness. As a result, an abrasion resistive property (α value) will be lowered, and variation of surface roughness of the photoreceptor will be induced by high coverage printing. In addition, a charge transport agent having a high charge transport property has a large molecular size, and it exhibits low compatibility with a curable binder resin. Therefore, a local charge failure will be produced by aggregation or crystallization of the charge transport agent. It will produce a problem of generating an image fault (a spot fault) by this local charge failure.